This invention relates generally to pipeline monitoring systems and more particularly concerns a sensor for detecting passage of a xe2x80x9cpigxe2x80x9d through a pipeline.
Operators of pipelines use pigs in a variety of activities including the separation of different liquids or gasses as they are conveyed through the pipeline and cleaning the pipeline of foreign materials. Generally, the pipeline operator uses mechanical or xe2x80x9cdumbxe2x80x9d pigs for such applications. Sensing the passage of xe2x80x9cdumbxe2x80x9d pigs through pipelines is historically accomplished by mechanical means. Intrusive threaded adapters are welded in place with spring-loaded shafts extending into the pipe. The shafts are temporarily deflected by the pig as it moves through the pipe and a spring-loaded lever or flag is released to give a visual indication of the pig""s passage. By locating many of the intrusive pig xe2x80x9csignalersxe2x80x9d along the pipeline, the operator can monitor the progress of a pig through the line by the sequential release of the flags. Occasionally, operators need to ascertain the condition of their pipeline. xe2x80x9cDumbxe2x80x9d pigs are not useful in these applications so active xe2x80x9csmartxe2x80x9d pigs with sensors and recording means are used. xe2x80x9cSmartxe2x80x9d pigs have a circumferential array of sensors which spring radially to snug them against the inside of the pipe to measure the pipe wall thickness about every eighth of an inch as the pig moves through the pipe.
Intrusive adapters are fraught with problems. For one, an annular high-pressure rotary or sliding shaft seal is necessary to prevent product inside the pipe from escaping as the shaft moves in either direction. Moreover, while many products carried by the pipeline are not corrosive to steel, they do make the annular high-pressure rotary or sliding seal of an intrusive signaler very difficult to achieve. Another problem is that, in preparation for the next pig passage, the operator expends considerable field time manually compressing the flag springs and ensuring that their detents are properly positioned to hold the springs compressed. To reduce the field time required to monitor the system, a multi-contact electrical switch could be actuated by the flag to power a local elevated incandescent signal lamp and/or possibly a remote readout panel. Even so, before the next pig passage, the operator would still have to return to each site to compress the flag spring, check the signal lamp and record any failures or malfunctions in his log. A further problem with intrusive adapter systems is that pipeline terrain is often mountainous, arid or subsea with exposure to ice heaves, mud slides, earthquakes, hurricanes, lightening, forest fires and other hazards that could damage signalers previously logged as fully functional. Another serious problem is that gaseous products separated by xe2x80x9cdumbxe2x80x9d pigs are a hazard to the spring-loaded shaft because the pigs often become temporarily stuck on welds in the pipe or at low spots along the line. Pressure slowly builds behind the stuck pig and eventually when it becomes unstuck the pig is, for a distance of several hundred yards, accelerated to speeds much faster than the average speed of the product. The shafts of the signalers are, therefore, sometimes sheared off as the high speed pig encounters them. Whatever the cause, if any of the welded threaded adapters are defective, maintenance or replacement involves great expense. The pipe must be exposed so that the defect can be viewed. Chippers remove the protective corrosion coating, grinders gently remove some of the smaller defects, cutting torches remove some of the larger defects and welders reweld the original threaded adapter to the pipe. If the original threaded adapter is not reusable, it is removed from the pipe with a cutting torch and its replacement is welded to the pipe. After approval of the work by a quality control group, the pipe must be sandblasted and coated with anti-corrosive material.
The use of xe2x80x9csmartxe2x80x9d pigs introduces additional problems. For example, xe2x80x9csmartxe2x80x9d pigs are very rigid and can only tolerate roughly a 20% reduction in pipe diameter. Consequently, all of the intrusive components must often be removed from the pipeline to prevent damaging the xe2x80x9csmartxe2x80x9d pig and also to prevent the xe2x80x9csmartxe2x80x9d pig from damaging the intrusive signaler. In xe2x80x9csmartxe2x80x9d pigs which use magnetic sensors, the magnets are so strong that they saturate the magnetizable steel pipe wall so that, as the xe2x80x9csmartxe2x80x9d pig moves its magnets beyond the previously saturated steel pipe regions, the regions do not return to zero magnetization but retain roughly 20% of the magnetization. For magnetic sensor xe2x80x9csmartxe2x80x9d pig systems, the industry standard signal frequency of 22 Hz adopted about 20 years ago for transmitters is lower than power line frequencies of 50 Hz and 60 Hz and lower than the first subharmonic for line powered cathodic protection systems at 25 Hz and 30 Hz. However, some European electric railroads use 50 Hz/3, or 16.6 Hz with a first subharmonic at 8.3 Hz. Therefore, this frequency results in a significant noise problem which is barely addressed by improved active filters and algorithms. For systems with their antenna close to the pipe, the noise ratio at 22 Hz is well beyond the capabilities of the lower power, battery powered, active filters required by the industry. This signal to noise ratio gets worse as the system operating frequency is lowered towards static or DC because of the electrical railroad frequency at 16.6 Hz/8.3 Hz, the AC components of the cathodic protection systems at 50 Hz/25Hz, the DC components of the same systems and the static residual 20% magnetization after xe2x80x9csmartxe2x80x9d pig runs.
The industry""s standard portable non-noise canceling single antenna for sensing industry standard 22 Hz transmitters and its associated waveforms are illustrated in FIGS. 1 and 2. Since the industry standard transmitters generate an AC magnetic flux signal they can be sensed by the industry standard antennas when they are motionless in a magnetic steel pipe. This function can be done with either a clockwise or counterclockwise winding equally well, and the operator is free to inadvertently reverse the phasing of the induced EMF e1 by rotating the entire antenna A1. Consequently, while known sensors are used to indicate the presence of a pig in a pipeline, the direction of motion of the pig in the pipeline is not indicated by the sensor. For clarity, all of the physical components in FIG. 1 are shown in cross sectional views along their centerlines except for the permanent magnet PM and the last half turn of the winding W. The last half turn completes the output circuit and contributes to the induced EMF e1. This permits the magnetic flux lines F1, F2 and F3 to be drawn on the surface of the cross section where they are inside the entity. In the production antenna winding, thousands of turns of fine copper magnet wire are used to provide the needed sensitivity. However, in FIG. 1, the winding W is shown as a uniformly spaced solenoid of magnet wire to permit the details of the core C, winding W, magnetic fluxes F1, F2 and F3 and induced EMF e1 to be graphically presented. To minimize perturbations in the flux paths F2 and F3 threading through the magnetic steel core C of the antenna A1, a winding W and a core C of equal length L1 are used. This fully distributed winding W further makes the axes of symmetry Z1 and zero crossing Z2 of the antenna A1 coincident and perpendicular to the axis of movement X of the permanent magnet PM. If there were no permanent magnet PM and no pipe P, the residual magnetic flux F3 from the magnetic core C of the antenna A1 would close upon itself in a static fashion, thus inducing no EMF. If there were no pipe P and a permanent magnet PM moved uniformly in relation to the antenna A1, when the centerline of the permanent magnet PM crossed the axis of symmetry Z1, for each positive induction in the winding W by each line of dynamic magnetic flux F2 there would exist by reason of symmetry an equal and opposite induction so that the net induction would be zero. Looking at FIG. 2, with no pipe P, the induced EMFs are at their maximum as shown in the no pipe plot PNO. All these EMFs must have symmetrical positive and negative areas separated by five zero crossings O1, O2, O3, O4, and O5 which is characteristic of this non-noise canceling antenna A1. The peak positive and negative inductions for any material in the core C would occur on either side of the axis of zero crossing Z2, even for magnetic material with zero magnetic resistance or an air core. With a pipe P and with a permanent magnet PM uniformly moving in a positive direction Mp and having north N and south S polarity as shown, it may not be apparent that the peak positive or negative inductions would still occur just before or after the centerline of the permanent magnet PM crosses the axis of zero crossing Z2, providing they share the same axis of symmetry Z1. The pipe material and its geometry would only slightly influence the location of the axis of zero crossing Z2 as long as there is a compensating variation on each side of the axis of symmetry Z1. However, the amplitudes of the peak inductions for both positive and negative movement directions would be strongly influenced by the pipe material and its geometry. The above analysis has been experimentally confirmed. The EMF plot PTK results for thick pipe and for a permanent magnet PM moving in the positive direction MP. The EMF plot PTNP results for a thin pipe with the permanent magnet PM moving in the positive direction MP. The dashed EMF plot PTNN results for thin pipe with the same permanent magnet PM moving in the negative direction MN. The axis of zero crossing Z2 for the permanent magnet PM would be the location of peak coupling for a transmitter. This is the industry standard arrangement for magnetic transmission through a magnetic pipe to a magnetic core antenna. This arrangement also introduces problems to the pig monitoring systems. Since there is no noise canceling in this antenna A1, the induced EMF e1 requires the use of active filters to improve the signal to noise ratio before it can be assessed to determine whether it qualifies as a legitimate transmitter event. As earlier discussed, the transmitter and the active filter operate at the industry standard 22 Hz. The typical design spacing S1 between the axis of the antenna X1 and the outer surface POD of the magnetic pipe wall used throughout the industry is about two feet minimum. However, in many applications, the pipe P is buried twenty or more feet below ground level. Making an antenna sensitive enough to operate at a spacing S1 of twenty feet requires thousands of turns of magnet wire. Consequently, when the spacing S1 is reduced to about two feet, the noise induced by the flux F3 in these thousands of turns begins to overload the active filters and the distinction between noise and signal cannot be reliably maintained. Noise is, therefore, often mistaken for a transmitter signal.
Even with proper phasing and no pipe, so that no medium magnetic flux F3 is present to generate any noise and no filter is needed, known sensors still have directionality problems. Assume that a constant medium strength 22 Hz transmitter is so remote that it does not induce any appreciable EMF in the winding W and that its steady movement is towards the axis of zero crossing Z2. The steady movement of the transmitter will steadily increase the magnitude of the induced EMF e1 until the transmitter location of peak coupling is reached at the axis of zero crossing Z2. As the transmitter continues to move away from the axis of zero crossing Z2, the magnitude of the induced EMF e1 now declines through the same magnitudes previously induced as it moved towards the axis of zero crossing. This symmetry in magnitudes of the induced EMF e1 is characteristic of any single antenna scheme. Consequently there is no distinguishing between the two directions for a transmitter.
It is, therefore, an object of this invention to provide a noise canceling dynamic magnetic flux sensor which is non-intrusive to the monitored pipeline. Another object of this invention is to provide a noise canceling dynamic magnetic flux sensor which eliminates the need for seals between the sensor and the pipeline wall. A further object of this invention is to provide a noise canceling dynamic magnetic flux sensor which eliminates the shearing of indicator components of the sensor. Yet another object of this invention is to provide a noise canceling dynamic magnetic flux sensor which reduces the likelihood of physical damage to the sensor or the pig. It is also an object of this invention to provide a noise canceling dynamic magnetic flux sensor which reduces the maintenance requirements for the sensor. Still another object of this invention is to provide a noise canceling dynamic magnetic flux sensor which eliminates the need for manual resetting of in-field indicators between pig runs. An additional object of this invention is to provide a noise canceling dynamic magnetic flux sensor which reduces the need for in-field inspection of signaler conditions. Another object of this invention is to provide a noise canceling dynamic magnetic flux sensor which minimizes the size of the sensed magnet so as to operate in small magnetic pipe. A further object of this invention is to provide a noise canceling dynamic magnetic flux sensor which effectively cancels noise imposed on the sensor, including railroad frequency and cathodic protection system frequency noise and is non-responsive to cathodic protection system DC component noise and static residual pipeline magnetization noise. Yet another object of this invention is to provide a noise canceling dynamic magnetic flux sensor which eliminates the need for active filters to improve noise ratios in the detected signals. It is also an object of this invention to provide a noise canceling dynamic magnetic flux sensor which reduces the impact of spacing variations between the sensor and the pipeline. Still another object of this invention is to provide a noise canceling dynamic magnetic flux sensor which is adjustable in the field to account for the specific noise characteristics of the sensor location. A further object of this invention is to provide a noise canceling dynamic magnetic flux sensor which can distinguish direction of movement by transmitters and permanent magnets in magnetic steel pipe.
Noise canceling can be achieved by connecting, in series and out of phase, a pair of inductive sensors, identically constructed and symmetrically situated, so that the symmetrical noise magnetic flux from the pipe threading through a portion of the winding of the first sensor will identically thread through a portion of the winding of the second sensor as it returns to the pipe. An inductive magnetic sensor is not responsive to DC noise and, by adding turns to the winding, the system detects small permanent magnets moving slowly through large diameter magnetic pipe with thick walls. Any change in the symmetrical noise magnetic flux from the pipe induces equal and opposite EMFs in the identical portions of the two identical windings. This passive cancellation of these two EMFs creates a virtual noise ground node somewhat like the active virtual ground created at the inverting input terminal of an operational amplifier configured as an inverter. This noise cancellation is not derived from the properties of the materials along the path of the threading magnetic flux but only from the symmetrical disposition of the materials about the axis of symmetry. This noise cancellation is not related to frequency and is fully operative from near static to near RF frequency range. Preferably, magnetic steel cores are used to enhance the EMF induced in the antennae. The magnetic core material minimizes the end effects that occur in any solenoid winding by providing an alternate route of lower magnetic resistance for any magnetic flux passing through it than would air.
Accordingly, a noise canceling dynamic magnetic flux sensor is provided for detecting the passage along the centerline of a magnetic steel pipeline of a pig containing a permanent magnet or transmitter. The sensor has a noise-canceling inductive array for sensing the dynamic non-symmetrical signal magnetic flux resulting from passage of the permanent magnet or transmitter. The array has first and second substantially identical magnetic steel cores in end-to-end spaced-apart alignment on a common longitudinal axis and first and second substantially identical inductive coils uniformly wound about the first and second cores, respectively. The coils are wound for symmetry of the sensor about an array axis which is perpendicular to the longitudinal axis of the cores and bisects the space between the cores. The coils may be wound in either clockwise or counterclockwise directions with their lead wires interconnected to establish the out-of-phase noise canceling of the symmetrical noise magnetic flux from the pipe. The cores and coils are secured externally of the pipe with the longitudinal core axis parallel to the pipe centerline. The sensor has a first circuit electrically connected to the coils for detecting at least non-symmetrical portions of a Faraday induced electromotive force across the coils and a second circuit responsive to the detecting circuit to generate electrical signals indicative of the passage and direction of a permanent magnet or transmitter across the array axis.